Good to hear you're back :)

Your machine work area is quite small, so it may be OK, but I'd seriously consider using 20mm rail compared to 15mm. When you see both, it looks quite a lot bigger which in general makes the design easier. I've got some spare hiwin HGH15, but haven't used it partly due to the size.

As to the question of the holes, I would ask the seller as they can probably cut it to whichever you prefer. The spare 760mm pieces I have have the holes centered, so 20mm from both ends.